The primary objective of the proposed study is to determine whether working at night is associated with decreased levels of the nocturnal urinary concentration of 6-sulphatoxymelatonin, and increased levels of luteinizing hormone (LH), follicle stimulating hormone (FSH), and estradiol in a sample of healthy women of reproductive age. Secondarily, this study is designed to investigate whether urinary levels of melatonin are lower and levels of reproductive hormones are higher during daytime sleep relative to nighttime sleep among women who work at night. Approximately 200 nurses who work the night shift exclusively and 150 nurses who work the day shift exclusively will be recruited as volunteers in the Seattle metropolitan area. Eligible participants must be between the ages of 20 and 40, employed for at least 20 hours/week, resident of King or Snohomish County, do not take oral contraceptives or other hormone preparations, and have no personal history of breast cancer. In addition, the night shift nurses must normally sleep at night during off days. This study will employ a design which allows for both between-subject comparisons of night shift v. day shift workers, and within-subject comparisons during day sleep v. night sleep among the night shift workers. Participation will span two months. In the first month the study will evaluate menstrual cycle regularity, determine the day of ovulation using a commercial kit, conduct a personal interview regarding employment and reproductive history, assess the participant's ability to adjust to shift work, and collect a blood sample. In the second month urine sample collections will be scheduled during both work and sleep periods, and will take place in the early to mid-luteal phase of each participant's cycle. Sleep patterns will be measured via actigraphy during the sleep periods corresponding to urine collection. Oral temperature data will be collected during wake-time in the night shift workers to determine temperature amplitude. Information will be collected on factors which may be related to the hormones under study, such as alcohol consumption, medication use, and hours of daylight during sample collection. Urine samples will be assayed for the primary urinary metabolite of melatonin, 6-sulphatoxymelatonin, and levels of LH, FSH, and estradiol.